Correlations between the irradiated dose to lymph node regions and lymph node recurrence when involved field radiotherapy used for limited-stage small cell lung cancer

Objective: To analyze the relationship between the radiation doses to mediastinal lymph nodes regions and the regional failure patterns when involved field radiation therapy (IFRT) was used for limited-stage small cell lung cancer (SCLC). Methods: The mediastinal lymph node regions (group 1 to 10) of the limited-stage SCLC patients received definitive radiotherapy were contoured in treatment planning system. The intentional or incidental radiation doses to each lymph node regions were recorded. In-field recurrence, marginal recurrence and out-of-field recurrence were respectively defined as the volume of failed lymph nodes located within the 80% iso-dose lines, in the 80%-20% iso-dose lines and beyond the 20% iso-dose lines of prescribed doses. Results: A total of 1216 lymph node regions in 76 patients were contoured. The median follow-up time was 17.4 months. At diagnosis, lymph node regions with metastatic rates >50% were 4R (68.7%), 4L (57.9%), 10R (57.9%), 2R (56.6%) and 7 (51.3%). The positive lymph node regions were all subjected to prescribed doses. The lymph node regions that received incidental radiation doses of more than 3000 cGy were: 3P, 4L, 7, 6, 4R, 5, 2L. The median lymph node failure-free time was 9.8 months. In this study, only 1 patient developed out-of-field mediastinal lymph nodes failure. The rest of out-of-field recurrences and marginal recurrences were developed in the supraclavicular regions or contralateral hila. Conclusions: When IFRT is used to treat mediastinal lymph node regions for patients with SCLC, negative mediastinal regions can be subjected to considerable incidental radiation doses. Out-of-field recurrences of the mediastinal lymph node regions are rare. This is contributed by the incidental radiation dose to these regions. (authors)